The Universe Is Expanding, But How Fast Is Up For Debate

In the 1920s, astronomer Edwin Hubble brought about the modern age of cosmology when he discovered that the universe is expanding at a predictable rate, which has since been called the Hubble constant. Nearly 100 years later, more precise measurements have sharpened his accuracy—but may also put our current understanding of physics in limbo.

We Live In A Raisin-Bread Universe

At the turn of the 20th century, the hot topic in cosmological circles was nebulae. We could see them with telescopes, but we didn't know how far away they were—were they interstellar clouds within the Milky Way, or were they far-off galaxies of their own? If you've ever wondered whether a light in the night sky is a satellite or a star, you know how difficult it is to tell the distance of things in space. In the early 1900s, Henrietta Swan Leavitt discovered what became known as Cepheid variables, a type of star whose brightness varied at a rate that could be used to calculate their absolute luminosity, or intrinsic brightness. That's important because astronomers can use a star's luminosity to measure its distance from us. A decade or so later, Edwin Hubble realized that many nebulae contained these variable stars, and could therefore make the important discovery that nebulae weren't located in our own galaxy, but far beyond it, existing as galaxies in their own right.

Oh, but that's not all. Next, Hubble compared these distance measurements with each galaxy's velocity, and found that the further away the galaxy was, the faster it was moving away from us. That led to the bombshell of the century: the universe was expanding. (To understand why more distant galaxies are moving faster, imagine a loaf of raisin-bread dough. When you put it in the oven, all of the raisins are evenly distributed, but as it rises in the oven, the raisins near the edges move outward faster than those in the center.) Hubble's formula to determine the speed of a galaxy, called Hubble's law, is v = H0d. H0 is, you guessed it, the Hubble constant, which astronomers have used ever since to judge the rate at which the universe is expanding.

Think of the universe as a baking loaf of raisin bread, and galaxies as the raisins. As the dough rises, the raisins on the outside move outward faster than the raisins on the inside.

Trouble In Paradise

When Hubble made this discovery, technology was not what it is today, to put it mildly. As a result, Hubble's estimate for the value of the Hubble constant was pretty imprecise. One big reason for launching the Hubble Space Telescope in the 1980s was to get a more precise estimate for a number that at that time was somewhere between 50 and 100 km/sec/Mpc (kilometers per second per Megaparsec)—they wanted to whittle the accuracy down to at least 10 percent, which is still a wildly imprecise margin for science. Fast forward 30 years, and even more precise instruments such as the Wilkinson Microwave Anisotropy Probe (WMAP) honed the number to 69.3 km/sec/Mpc. Then in 2013, the expansion rate of the universe put on the brakes when the Planck satellite used background radiation from the Big Bang to find that the Hubble constant was closer to 67 km/sec/Mpc.

But in December 2016, a group called H0 Lenses in COSMOGRAIL's Wellspring, or H0liCOW (get it?), used Einstein's theory of general relativity to determine that the Hubble constant was a much faster 72 km/sec/Mpc. Despite its name, improved technology means that the Hubble constant keeps changing. What does that mean? A lot. It could mean that there are yet discovered elementary particles at play. It could mean that dark energy, which was previously blamed for shifts in the expansion rate, isn't there at all, and is instead a theoretical form called phantom energy. This all would mean new physics, and a drastic change in our understanding of the universe. But for now? It's too soon to tell.